Power is energy flowing! It is measured as energy per unit of time
- Energy is in Joules (J)
- Power is in Joules per second (J/s) which has the special unit of Watts (W)
1 Watt = 1 Joule of energy per second
1 W = 1 J/s
No, not that type of Power Formula!
Power is the rate of energy per time:
P = Energy Time
Example: 3000 J of energy is used in 20 seconds, what is the power?
P = 3000 J 20 s = 150 W
We can also change the formula this way:
|P = Energy Time|
|Energy is also Force × Distance, so:||P = Force × Distance Time|
|Distance/Time is Velocity, so:||P = Force × Velocity|
Example: These guys pull with a combined force of 4000 N, and manage a velocity of 0.4 m/s, what power are they using?
P = 4000 N x 0.4 m/s
= 1600 N m/s
= 1600 W
Hang on! Is N m/s (Newton meter per second) really the same as W (Watt)?
In fact all these are the same:
|Start with:||N m/s|
|A Newton is kg m/s2:||kg m/s2 m/s|
|A Joule is kg m2/s2:||J/s|
|A Watt is J/s:||W|
1 horsepower (hp) is 745.7 W (or 746 W for electrical motors)
Example: Sam's car has a 200 hp engine. What is that in watts?
200 × 745.7 = 149,140 W = 149 kW (to nearest kW)
What is a kiloWatt Hour?
1 kWh is the Energy used when the Power is 1 kW for 1 hour.
This is a little 1 kW radiator. How much energy does it use in an hour?
- 1 kW is 1000 Watts which is 1000 J/s
- There are 3600 seconds in an hour
So in an hour it uses:
1000 J/s × 3600 s = 3,600,000 J = 3.6 MJAnd so 1 kWh (kiloWatt hour) is 3.6 MJ of Energy